Here's a scenario: New home buyer-client comes to me to find a house. My first question to them is always "how important is it that you can walk to the train and to shopping?" My second question is then "Have you been pre-qualified for a mortgage?". If it's from a lender I'm familiar with we're all set to go look at houses. If it's from a questionable company I may ask that they be "re-qualified" by a lender I've worked with before.
Most buyers are happy to have another company with which to compare rates, but some buyers balk at the prospect of having their credit pulled again because they fear it will damage a hard-earned high credit score. This is a ploy that some mortgage guys use to stop the buyer from shopping around for a better rate. Read this fantastic article by loan officer Dan Green (whom I've actually never worked with) to understand more about how much this will truly damage your credit score.